1. Field of the Invention
The present invention relates to a dispersion composition for a magnetic display which can display and erase a sharp record by magnetism.
2. Description of Related Art
A dispersion composition for a magnetic display is generally known. Such a dispersion composition typically utilizes a magnetic force to cause the migration of magnetic particles and to thereby accomplish a display. Known dispersion compositions generally comprise: magnetic particles, a dispersing medium, a thickening agent, and if necessary, a colorant.
A recording is formed by employing a writing panel including a front substrate containing the dispersion composition and a head or the like provided with a magnetic pen or an electromagnet. In forming the recording, the magnetic particles are attracted by the magnetic force of the magnetic pen or the electromagnet and migrate from a back substrate of the writing panel. A display is formed by the difference between the color of the dispersion and the color of the magnetic powder or particles. In addition, when a magnetic force is produced from the back substrate by a magnetic eraser, the magnetic particles attracted toward the front substrate are pulled toward the back substrate, so that the displayed record can be erased.
In general, a dispersion composition for a magnetic display can be used in a writing plate or the like in which the dispersed magnetic particles are attracted by a magnetic pen to display letters or patterns as they are recorded. For example, the dispersion compositions for magnetic displays have been used in various fields of toys, writing implements and the like. They are usually used under circumstances of daily life at variable temperatures and variable humidity levels rather than under constant environmental conditions.
In conventional dispersion compositions for magnetic displays, a vinyl chloride sheet or a polyester sheet is generally used as the substrate of a display panel as described above. During weather conditions exhibiting low temperatures and dry air, static electricity is easily generated; thus, an electrification phenomenon takes place. Further, under substantially any temperature and humidity condition, when a magnetic pen used with a magnetic display is moved to depict letters or patterns, the front substrate or the back substrate builds up charge due to the generation of the static electricity by the movement of the magnetic particles and collisions between the magnetic particles. Thus, static electricity is also generated by the friction between the magnetic tip of the magnetic pen and the surface of the front substrate, or by the friction between the magnetic eraser and the surface of the back substrate. In addition, when the magnetic pen is held manually, static electricity is also generated by the friction between the surface of the front substrate of the dispersion composition and the operator's hand.
When an electric charge is built up as described above, the magnetic particles become adsorbed on the front substrate or the back substrate, so that the display portion of the panel for magnetic display becomes soiled. Thus, the contrast between written lines and unwritten portions deteriorates. Moreover, when an erasure of written lines is attempted, an erasure defect occurs such that the written lines cannot be completely erased. Furthermore, since the dispersion system of the thickening agent and the colorant is broken down by electrostatic attraction, the magnetic particles sediment and phase separation occurs; thus the durability of the magnetic dispersion composition deteriorates and the sharpness of the letters or the patterns is consequently lost.
In order to solve these problems, various techniques have been suggested. For example, an antistatic agent can be applied onto the surface of the front substrate. Alternately, an antistatic synthetic resin film can be integrally mounted on the surface of the front substrate so that discharging is promptly accomplished through the antistatic synthetic resin film to prevent charge build up when static electricity is generated.